SUBSURFACE GEOLOGY IN THE NORTHERN SAN FRANCISCO BAY REGION, CALIFORNIA

Exploration for oil and natural gas during the past 60 years in southern Sonoma and Napa counties and westernmost Solano County has provided data that, when evaluated by the established methods of petroleum geology, can augment significantly the results of surface geological mapping and potential-field studies. On electric logs from nine or more exploratory wells extending from near Sebastopol southeast some 40 km through the Cotati and Petaluma valleys, interpreted correlative intervals on the resistivity curves may represent cycles of estuarine, fluvial and lacustrine deposition in the Petaluma Formation. On Island No. 1, northeast of San Pablo Bay, a cluster of wells defines tight, northwest-trending folds in Miocene and older strata overlain unconformably by nearly undeformed Pinole Tuff (5.2 – 5.4 Ma). A correlation section relating those wells to wells and outcrop to the south-southeast in northern Contra Costa County and to the north-northwest in westernmost Napa County illuminates the complex stratigraphic relationships between those two areas; in the northern area, where Weaver (1949) assigned a thickness of 500 ft to the Huichica Formation, well data show it to be 3500 ft thick. Integration of these several data sets, plus recent radiometric dating of volcanic rocks in Sonoma County, provides the basis for significant revision of geological cross-sections (Wright and Smith, 1993) across the major gravity low beneath San Pablo Bay. Incorporation of subsurface data from several wells in the tightly compressed syncline between the Rodgers Creek fault and the Tolay fault at Sears Point anticline suggests yet another interpretation of the complex faulting in that area.